Marathon legend Joan Benoit Samuelson in her 1987 autobiography Running Tide, Chapter 6, comments on how it feels to run well:

... I left the starting line with a confidence that is hard to describe. It isn't euphoria, exactly, and it isn't overconfidence. It's as if I'm an inventor; I created this body, and now I'm watching it work. Any glitches in the moving parts? (No.) Are the pumps and valves leaking? (No.) Is there too much stress anywhere? (Not yet.) The invention can be monitored for just so long before the creator either begins to trust it or watches it break down. There's a point in every race, and it's different in each, where I realize that my body is either going to make it to the end in fine style or be in trouble. Once that point is passed I start making decisions to account for the condition of the machine. ...